Coil electrode



y 1947- R. H. STUART 2,424,518

COIL ELECTRODE Filed Dec. 4, 1944 f N vE/v 7-0/2: FA YMUND H. 5 TUAA T H5 A T TUPNEY Watented July 22, 1947 2,424,518 con. nmic'monn Raymond H. Stuart, Cleveland, Oliio, casino: to General Electric Company, a corporation of New York Application necembei- 4, 1944, Serial No. 566,803

8 Claim.

This invention relates to filamentary or wire coil structures suitable for electric lamps and for discharge electrodes, including coil cathodes for lamps or tubes of positive column low pressure type, which are characterized by a difiuse discharge that substantially fills the tube. The invention is hereinafter explained with particular reference to activated coil electrodes of refractory metal for ordinary tubular, positive column, fluorescent lamps, though it may, also be found useful for any lamp coils that carry a filling or charge of activating or emissive material or the like.

of powdery metal compound(s), such as mixtures of alkaline earth oxides including those of barium and strontium. In order to charge the coil therewith, powder for the filling may be suspended in a liquid organic binder such as a lacquer of low viscosity nitrocellulose. The suspension forms a sort of cream, which is applied to the coil and then allowed to dry. The application of the emission mix to the coil may conveniently be effected by immersing or dipping the coil in the fluid mix as described in U. S. application Serial No. 488,955 of John Flaws, Jr., filed May 29, 1943, now Patent No. 2,363,055, or in Serial No. 548,852 of William P. Zabel, filed August 10, 1944, both assigned to the assignee of this application. Very commonly, the powder thus suspended in liquid to form an emission mix consists not of the desired oxides, but of other alkaline earth components such as carbonates or hydroxides. During exhaustion of the lamp, such compounds are converted into the oxides by suitable heat, which also serves for decomposing the binder (if any is used) and eliminating its residual solids. This heat is commonly produced electrically, in ways which may involve passage of electric current through the filament.

It is desirable that the filling in a coil should amount to a plentiful supply of the material, and should occupy as great a length of the coil as may be without extending right to the support or lead wire, or to its filament clamp. Generally, it is desirable to keep the material away from each of the lead wires or clamps. Because it is impracticable to heat the short coil segment next to the clamp hot enough to decompose the alka- Fillings such as here referred to may consist 2 ed components of the emission mix completely during exhaust, it follows that filling material in this segment of the coil survives more or less unchanged in the finished lamp, and is afterward decomposed to yield deleterious gas during the operation 01' the lamp. Yet proper control of the extent of the filling in the coil is in practice dimcult: no matter how carefully it is applied,

the liquid filling material sometimes runs along in the coil right to the clamp, and sometimes stops an excessive distance short of the clamp.

I have found a way of definitely controlling and limiting the spread of the filling liquid along the coil toward either or both coil ends or lead wire clamps, and of thereby assuring a maximum amount of activating material in the coil without risk of its extending into the short coil portions right next the clamps, where it cannot be heated. This I accomplish by deforming the coil between the clamp and the desired limit of approach of the filling toward the clamp, in such a way as to reduce or substantially nullify the capillary or surface tension elTect that permits or promotes the spread of the filling along the filament. In other words, the coil is locally distorted near the wire clamp into a form offering greater resistance than the undistorted coil to the travel of liquid lengthwise or the coil. The deformation may involve the whole coil segment adjacent the clamp from which it is desired to exclude the emission mix, or may only include the outer portion of this coil segment, remote from the clamp. The deformation can be effected very conveniently and economically, without necessity for any extra operation or expense. Various other features and advantages of the invention will appear from the description of a species or form of embodiment, and from the drawings.

In the drawings, Fig. 1 is a tilted side view of a coiled filament mount embodying the invention; and Fig. 2 is a fragmentary side view of one end of the filament coil and the corresponding lead-wire clamp, on a larger scale than Fig. 1.

Fig. 3 is a similar view of the coil end and clamp prior to charging of the coil with filling such as emission mix, on a larger scale than Fig. 1; and Fig. 4 is an end view of part of the coil taken as indicated by the line and arrows 44 in Fig. 3.

line earth compounds and eliminate the unwant- .Fig. 5 is a view similar to Fig. 3 showing the 3 filament coil and the lead wire clamps prior to closure of the clamps and deformation of the filament coil for the purposes of my invention, with a diagrammatic profile outline of clamping jaws for closing the clamps; and Fig. 6 is a somewhat diagrammatic tilted view of the clamping jaws when open quite wide.

Figs. 7 and 8 are views similar to Figs. 3 and illustrating a different deformation of the filament for the purposes of my invention.

Fig. 9 is a somewhat diagrammatic fragmentary side view illustrating details of a cathode coil of special construction that may be used in the devices illustrated in Figs. 1 to 8.

Fig. 1 shows a mount 1 comprising a flare or stem tube 2, an exhaust tube 3, and current lead wires 4, 4 which extend. through the stem 2 and the seal 5 at its inner end, through which the exhaust tube 3 opens. At the inner side of the seal 5, the leads 4, 4 are spread and bent to form hooks 6, 6 in which are clamped the ends of the coiled refractory metal filament 1, preferably of tungsten wire. As described in U. S. Patent No. 2,312,245 to John Flaws, Jr., the extremity 8 of each lead wire 4 beyond the clamp 6 may be bent to extend parallel to the coil 1 at one side thereof. As shown in Figs. 1 and 2. the Portion of the coil 1 between the clamps 6, 6 is filled or charged with activating material 9 as above described. When mounts I of this character are used in a gaseous electric discharge lamp such as the ordinary lowpressure positive column fluorescent lamp containing mercury and starting gas like argon, two of these mounts l are sealed into opposite ends of a tubular envelope, not shown, and their fllaments I serve as cathodes, while the lead wire extremities 8 serve as auxiliary anodes. The pressure of argon in such a lamp may amount to some 2 to 5 mm., and the pressure of mercury vapor during operation may be of the order of some 10 micron.

The liquid emission mix of finely divided material suspended in an organic binder is applied to a mid-portion of the coil 1 between the clamps 6,5 before the mount l is sealed into a lamp tube, preferably by immersing or "dipping" this portion of the coil in a supply of the liquid held in a suitable spoon, as it is called, as described in the above-cited Flaws and Zabel applications. The liquid "runs" or spreads in the coil 1 from where it is applied toward each of the clamps 6, 6. The extent to which the liquid spreads and it proximity of approach to the clamps 6, 6 depends on a number of factors which it is more or less difficult to control, such as the amount of liquid that the coil 1 takes up, the viscosity of the liquid, the composition of the nitrocellulose lacquer or other'binder used, the proportion of powdered solids to hinder, the temperature of the liquid mix from time to time, the evaporation of solvent or other liquid component (s) from the emission mix that has occurred when each coil is immersed in it, the presence and character of foreign matter on the tungsten wire of the coil, etc. These and other factors affect the capillary action or the surface tension of the liquid in the coil, and its spread or travel in the coil toward each of the clamps 6.

In order to control the approach of the liquid in the coil 1 toward either or both of the clamps 6, 6, I deform the coil as already mentioned at or suitably adjacent the clamp or clamps in question. As shown in Figs. 1, 2, and 3, this deformation l0 represents a distortion of the cross-configuration of the coil 1 acfiacent each clamp 6:

. 4 i. e., the circular helical coil is flattened to an elliptical or l00p-shaped elongated cross-configuration. The distortion at I0 may involve either a flattening of the coil convolutions individually from circular to elongated outlines, or a deflection of the convolutions into oblique, sloping positions relative to the coil axis, or both. As shown in Figs. 1. 2, 3, and 4, the flattening has been effected in the plane of the paper in Figs. 1, 2, 3; individual convolutions have been deflected to slope relative to the coil axis and also squeezed to elongated outlines; and the major axes of the elliptical or loop-shaped convolutions extend perpendicular to the plane of the paper in Figs. 1, 2, 3. The eflect of thus flattening the coil sections l0, l0 adjacent or close to the clamps 8, i is to increase the resistance of the flattened sections to travel of the liquid therein to such a degree that the liquid applied to the mid-portion of the coil I is virtually excluded from the sections l0, i0: 1. e., the flattened sections determine, limit, and define or bound the length of coil 1 which is filled or charged with emission mix, and which ultimately contains the activating oxides or material 9.

Figs. 5 and 6 illustrate one preferred way of producing coil deformations l0 such as shown in Figs.1-4. Fig. 5 shows a coil 1 in position in a lead wire hook 6 before the hook has been closed to clamp the filament, and also gives a fragmentary end-on view of suitable clamping jaws i I, II in engagement with the wire 4 at eitl' er side of the hook, in position ready to close the hook. As shown, each of the jaws H, II has a recess I! such as a V-groove of such depth as to accommodate part of the diameter of the wire 4, so that when the jaws have flattened and closed the hook 6 completely to grip and hold the wire of the coil 1, the jaws themselves are still separated a distance approximating, say, about half the diameter of the coil, in other words, a distance equal to the desired smaller dimension of the distorted coil section ill in Figs. 1, 2, 3. Thus the same action of the jaws H, Ii that closes the hooks 6, 6 to clamp the filament ends also flattens the cross-configuration of the coil 1 at IU. At the same time, incidentally, the jaws II, Il may also flatten the cross-section of the coil ends l3, l3 outside the hooks or clamps 6, 6. As shown, however, the surface of each jaw H at the inner side of the hook 6 is slightly higher than at the outer side, so that when the jaws come together, the coil section I0 is compressed rather more than the coil end I 3.

The degree to which the coil 1 needs to be distorted to stop the travel of liquid therein depends, in general, on the factors above mentioned, such as the amount of liquid on or in the coil, the viscosity of the liquid, etc., etc. In some cases, a mashing of the coil 1 to half its original diameter gives good results.

Fig. 7 is a view similar to Fig. 3 illustrating a somewhat different deformation of the coil 1 for the purposes of my invention. While individual convolutions are here shown as deflected to slope relative to the coil axis, with the effect of locally flattening the circular coil to a loopshaped, elongated cross-configuration, yet these individual convolutions are not changed from circular to elongated outlines to any considerable degree, if at all. In fact, the more important distortion ill in Fig. 7 consists in a spreading apart of at least two adjacent coil convolutions to such a degree as almost or largely to interrupt the helix as a path of travel of the liquid under the influence of capillarity or surface tension, with an incidental deflection of these convolutions and those adjacent each of them to positions of opposite and pronounced inclination relative to the coil axis, as well as of great divergence relative to one another. The virtual gap in the coil cylinder thus produced effectively stops the travel of liquid along the coil 1 toward the clamp B.

Fig. 8 corresponding to Fig. 5 illustrates one preferred way of producing a distortion I such as shown in Fig. '7 as an incident of closing a lead-Wire hook 6 to clamp the filament I. As here shown, one of the clamping jaws II, II at either side of the hook 6 not only has a V- groove recess I! of such depth as to accommodate part of the diameter of the wire 4. but also has alongside this groove a wedge or V-ridge M whose sides make an angle of some 45 to 60 more or less with one another. The coacting Jaw H need not necessarily have such a wedge H; as shown, indeed, it has a V-notch it to accommodate the wedge M. When the jaws II, II come together to flatten and close the hook 6 com pletely, the wedge M enters between two adjacent coil convolutions and spreads them against the sides of the notch IE to positions of wider divergence, stressing the metal of the coil I above its elastic limit and producing the permanent deformation above described, essentially as shown in Fig. '7.

In Figs. 1 to 8, the coil 7 is represented as a simple helical coil of plain tungsten wire. My invention is also applicable, however, to other types of coils, such as the coiled coil of tungsten wire now in common use in ordinary fluorescent lamps; or the coiled coil of tungsten wire with a loose tungsten wire overwind that is illustrated and described in U. S. Patent No. 2,306,925 to John O. Aicher, granted December 29, 1942; or the type of coil illustrated in Fig. 9, consisting of a simple coil of tungsten wire I having a close overwind iii of finer tungsten wire. For the plain coil 1 shown in Figs. 1-8, very favorable lamp life (measured in terms eith r of continuous operation, or number of cold starts endured without substantial end-blackening of the lamps) has been obtained in a 40-watt fluorescent lamp with coils of 146 turns of 3.5 mil tungsten wire i wound 273 turns per inch on a 23 mil iron man drel and having when finished a space of less than 1 mil between the adjacent turns, after completion of the mount. For the overwound coil l shown in Fig. 9, equally favorable or even better life (similarly measured) has been obtained in a. 40-watt lamp with coils of 114 turns (between clamps) of 3 mil tungsten wire I wound 222 turns per inch on a 22.65 mil iron mandrel after having been itself overwound 293 turns per inch with 0.7 mil tungsten wire Hi, the space between the fine wire on adjacent turns of the 3 mil wire being just a little over 1 mil after com-' pletion of the lamp mount. In'both cases, of course, the iron mandrels are removed (as by dissolving them out), after "setting the coils by the usual heat treatment, before filling or charging the coils with liquid emission mix.

In Figs. 59, various parts and features are marked with the same reference characters as those corresponding in earlier figures, in order to dispense with repetitive description.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electrode for electric discharge devices comprising a continuous wire coil extending substantially linearly between and secured to a pair of spaced lead-4n conductors, the portion of said coil intermediate said lead-in conductor being of such dimensions that a suspension of electronemissive material will spread along the coil by a capillary or surface tension effect and be n filled with the dried residue of a liquid suspension of powdered electron-emissive material, the end turns of said coil adjacent to the inner sides of said conductors being distorted to provide a barrier to the capillary flow of such liquid suspension and being free from said electron-emissive material.

2. An electrode for electric discharge devices comprising a continuous wire coil extending substantially linearly between and secured to a pair of spaced lead-in conductors, the portion of said coil intermediate said lead-in conductors being of such dimensions that a suspension of electronemissive material will spread along the coil by a capillary or surface tension effect and being filled with the dried residue of a liquid suspension of powdered electron-emissive material, the end turns of said coil adjacent to the inner sides of said conductors being flattened to provide a barrier to the capillary flow of such liquid suspension and being free from said electron-emissive material.

3. An electrode for electric discharge devices comprising a continuous wire coil extending substantially linearly between and secured to a pair of spaced lead-in conductors, the portion of said coil intermediate said lead-in conductors being of such dimensions that a suspension of electronemissive material will spread along the coil by a capillary or surface tension effect and being filled with the dried residue of a liquid suspension of powdered electron-emissive material, the end turns of said coil adjacent to the inner sides of said conductors being spaced by an opened coil turn from the said intermediate portion of the coil to provide a barrier to the capillary flow of such liquid suspension and being free from said electron-emissive material.

4. The method of manufacturing electrodes for electric discharge devices which comprises securing a continuous helical wire coil between a pair of spaced lead-in conductors and distorting the ends portions of the coil adjacent to the inner sides of said conductors to provide a barrier to the capillary flow of liquid coating material, and then applying to the portion of the coil intermediate said distorted ends a liquid suspension of powdered electron-emissive material.

5. The method of manufacturing electrodes for electric discharge devices which comprises securing a continuous helical wire coil between a pair of spaced lead-in conductors and flattening the end portions of the coil adjacent to the inner sides of said conductors to provide a barrier to the capillary flow of liquid coating material, and then applyin to the portion of the coil intermediate said distorted ends a liquid suspension of powdered electron-emissive material.

6. The method of manufacturing electrodes for electric discharge devices which comprises securing a continuous helical wire coil between a pair of spaced lead-in conductors and distorting the end portions of the coil adjacent to the inner sides of said conductors by opening up a turn of the coil to provide a barrier to the capillary flow of liquid coating material, and then applying to the portion of the coil intermediate said distorted ends a liquid suspension of powdered electronemissive materiel.

RAYMOND H. STUART.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Number 8 Name Date Kratky June 3, 1941 Gustin Aug. 2, 1932 Meeker et a1 Jan. 24, 1939 Spaeth Oct. 15, 1940 Mackay Oct. 12, 1926 Keyes Nov. 23, 1915 Flaws, Jr. Apr. 23, 1935 FOREIGN PATENTS Country Date Great Britain Jan. 13, 1941 France Mar. 28, 1927 

